First, I'd say that a single RAID5 disk for both the OS and everything else is a bad setup.

As others have mentioned, put the OS on a hardware RAID mirror using two drives. That one drive will be your root ZFS pool (rpool). (And if this were to be a long-lived server under my control, I'd create another two-disk RAID mirror for a second ZFS root pool (rpool2) to be used for OS upgrades and patches - always creating the new boot environment on the other rpool in order to avoid a nasty hell of ZFS rpool clones and snapshots. If the boot environment being updated is on rpool, the new boot environment is created on rpool2)

Then use the other 6 disks for the database - exactly how would depend strongly on what database and what it stores and how it's going to be used.

And yes, in general you will want to limit the ZFS ARC on a DB server - severely (you don't need to read /var/adm/messages very fast...). If your database isn't using ZFS to store data, there's no need for more than a token ZFS ARC, and especially for an Oracle DB not using ZFS storage an unrestricted ZFS ARC can cause severe performance problems. (Oracle DB tends to use large-page-size chunks of memory. ZFS ARC uses 4k pages. On a server with high memory pressure, dynamic Oracle memory demands will force the kernel to have to coalesce memory to create large pages for the Oracle DB process(es). ZFS ARC pressure then breaks those pages up - rinse, lather, repeat as the server unresponsively just sits and spins...)

HP Proliant? Meh. A few years ago, a customer I supported bought new HP servers - because they were "cheaper" than Oracle's servers. Oh? Well, the new servers weren't any faster than the old (so old they still had "Sun" on them...) servers - and it took quite a bit of BIOS tuning just to get the brand-spanking-new "fast" HP servers to even match the old Sun ones performance-wise. As far as "cheaper"? We had to install the HBAs ourselves (labor time is expensive...) and THEN we found out that the ILOM software wasn't part of the basic HP server - it had to be bought/licensed separately - then installed (even more expensive labor hours). Oh, and the HP server didn't come with four built-in 10 gig ethernet ports, so we had to add ethernet cards - more money and more time. When all was done, the customer paid a lot of money and wound up with new HP servers that took a lot of time and effort to make just as fast as the older Sun servers they replaced. Simply buying new servers from Oracle would have resulted in actually getting faster servers - for less money, less time, and a lot less effort.

Slapping a bunch of commodity parts around a good CPU and a decent amount of RAM doesn't make for a fast server. I/O bandwidth, memory bandwidth, disk controller quality? They matter too, and using the cheapest parts you can find in China slapped onto the cheapest motherboard doesn't cut it - especially when you turn around and nickel-and-dime customers over things like ILOM software licenses...

I'm not impressed with HP.</RANT>